1. Field of the Invention
The invention relates to methods of silica preform manufacturing, in particular for creation of optical fiber lightguides with reflective cladding deposited by microwave plasma enhanced chemical vapor deposition (PECVD).
2. Information Disclosure Statement
Optical fibers are currently manufactured through a drawing process, where fibers are drawn from a preform with a large diameter. These preforms are generally high purity glass or plastic. The fiber cladding is often applied to the preform prior to drawing the fiber.
The most common methods for the manufacture of fiber preforms involve chemical vapor deposition (CVD), which entail the use of vaporized raw materials that combine with oxygen and solidify into glass. The principle CVD methods can be grouped into two categories. The first is those methods that utilize thermal energy to create the precursor vapor, and includes modified chemical vapor deposition (MCVD), outside vapor deposition (OVD), and vapor axial deposition (VAD). The second utilizes electromagnetic radiation to ionize precursor gas, thus forming a plasma from which the glass is deposited. Method in this category include plasma CVD (PCVD) and plasma enhanced CVD (PECVD).
U.S. Pat. No. 6,138,478 by Neuberger et al discloses a method and device for silica preform production by microwave plasma deposition of an SiO2—F cladding on a silica rod. The invention uses microwaves with a frequency of 2,450 MHz. This method is limited in that it cannot produce a uniform deposition on a silica rod with a diameter greater than 25. This limitation is due to the nonsymmetry of E020 mode excitation and arising nonsymmetrical wave of TE type. Microwave power losses on irradiation of large diameter silica rods and a hole in the reactor can be up to 20% of the incident microwave power. This leads to deterioration of preform quality for large rods, and thus to a restriction on preform diameter. It is also impossible to increase productivity, as determined by deposition rate and silica rod diameter, by this method.
The closest analog to the present invention is disclosed in U.S. Pat. No. 5,597,624 by Blinov et al. A method of PECVD is described wherein a surface plasma wave of either the symmetric E01 or the hybrid HE11 type is excited on the outside surface of a dielectric starting body, such as a silica tube. However, this method cannot be used in commercial-scale manufacturing of large diameter and high quality silica preforms because of a lack of high power impulse microwave sources (both generators and amplifiers) that deliver microwaves in the 2450 MHz region with a 10 kW average power and 1 ms impulse duration.
The present invention is also useful for preventing hydrogen diffusion, or corrosion of the cladding due to environmental conditions, which can be especially severe in high temperature applications. Adverse environmental conditions combined with stress serve to exacerbate this problem. High optical losses due to hydrogen diffusion are found in known silica optical fibers. To prevent hydrogen diffusion, and thus protect the fiber and extend its useful life, a buffer SiOxNy layer is typically applied. Generally, the thickness of such a layer is in the range of 100–10,000 A depending on the optical fiber application. Other SiOxNy layer thicknesses, up to a few microns, can be produced if needed.
Although deposition of an SiOxNy layer is known and used to prevent hydrogen diffusion, modern sputtering or deposition equipment is expensive and these devices and methods fail to generate a homogeneous layer. Additionally, in present methods the deposition process is synchronized with the drawing of optical fibers from a silica preform. Applying an SiOxNy layer during drawing necessitates a decrease in the fiber drawing rate, and further results in a decrease in process productivity and an increase in the basic cost of fibers. This leads to considerable reduction of production efficiency (especially for preform diameters within 30–40 mm).
A method of depositing SiOxNy layers during the manufacture of optical fiber preforms, so as to prevent hydrogen diffusion, is known, and is described in Japanese Patent No. 62-65948 by Akira et al. This method eliminates the need to deposit SiOxNy layers during fiber drawing and thus eliminates the production efficiency problems described above.
However, this method requires a two-stage process and has a low efficiency in the deposition of chemical reagents (less than 50% of gaseous reagants are actually deposited). Two setups are required for use in this process. The first is a device for the deposition of a soot SiO2 layer by MCVD, VAD, OVD on a preform surface. The second is a device for vitrifying the soot in an atmosphere of N2 and He. This process is rather long, and Helium is expensive. The basic drawback of this method is the application of high temperature deposition technologies (MCVD, VAD, OVD) that do not produce effective N2 dissociation (even at temperatures exceeding 2000° C.) in a gas phase or effective N2-doping of synthesized SiO2 glass layers.